1. Field of the Invention
The present invention relates generally to a mobile communication system supporting packet data service. More particularly, the present invention relates to a method and apparatus for transmitting control information required for Hybrid Automatic Repeat request (HARQ).
2. Description of the Related Art
Universal Mobile Telecommunication Service (UMTS), which is a 3rd generation mobile communication system using Wideband Code Division Multiple Access (WCDMA) based on the European Global System for Mobile communications (GSM) system and General Packet Radio Services (GPRS), provides mobile subscribers or computer users with a uniform service of transmitting packet-based text, digitized voice, and video and multimedia data at or above 2 Mbps irrespective of their locations around the world.
Particularly the UMTS system uses a transport channel called Enhanced Uplink Dedicated CHannel (E-DCH or EUDCH) in order to further improve the packet transmission performance of uplink communications from a User Equipment (UE) to a Node B (interchangeable with a base station). For more stable high-speed data transmission, Adaptive Modulation and Coding (AMC), HARQ, shorter Transmission Time Interval (TTI), and Node B-controlled scheduling were introduced for the E-DCH transmission.
AMC is a technique of determining a Modulation and Coding Scheme (MCS) adaptively according to the channel status between the Node B and the UE. Many MCS levels can be defined according to available modulation schemes and coding schemes. The adaptive selection of an MCS level according to the channel status increases resource use efficiency.
HARQ is a packet retransmission scheme for retransmitting a packet to correct errors in an initially transmitted packet. Shorter TTI is a technique for reducing retransmission time delay and thus increasing system throughput by allowing the use of a shorter TTI than the shortest TTI of 10 ms provided by 3GPP Rel5. At present, 2 ms is under consideration as the length of such a shorter TTI.
Node B-controlled scheduling is a scheme in which the Node B determines whether to permit E-DCH transmission for the UE and if it does, an allowed maximum data rate and transmits the determined data rate information as a scheduling grant to the UE, and the UE determines an available E-DCH data rate based on the scheduling grant.
The Node B-controlled scheduling is performed such that the noise rise or Rise over Thermal (RoT) measurement of the Node B does not exceed a target RoT to increase total system performance by, for example, allocating low data rates to remote UEs and high data rates to nearby UEs. RoT represents uplink radio resources used by the Node B, defined asRoT=Io/No   (1)
where Io denotes power spectral density over a total reception band, that is, the total amount of uplink signals received in the Node B, and No denotes the thermal noise power spectral density of the Node B. Therefore, an allowed maximum RoT is total uplink radio resources available to the Node B.
The total RoT is expressed as the sum of inter-cell interference, voice traffic and E-DCH traffic. With Node B-controlled scheduling, simultaneous transmission of packets from a plurality of UEs at high data rates is prevented, maintaining the total RoT at or below a target RoT and thus ensuring reception performance.
FIG. 1 is a diagram illustrating a typical signal flow for data transmission and reception on the E-DCH. In the illustrated case of FIG. 1, a UE transmits uplink data on the E-DCH and a Node B performs Node B-controlled scheduling for the UE.
Referring to FIG. 1, the Node B and UE establish the E-DCH in step 102. Step 102 involves message transmission on dedicated transport channels. The UE transmits scheduling information to the Node B in step 104. The scheduling information may contain uplink channel status information including the transmit power and power margin of the UE, and the amount of buffered data to be transmitted to the Node B.
In step 106, the Node B monitors scheduling information from a plurality of UEs to schedule uplink data transmissions for the individual UEs. When the Node B decides to approve an uplink packet transmission from the UE, it transmits a scheduling grant including scheduling assignment information to the UE in step 108. The scheduling grant indicates up/hold/down in an allowed maximum data rate, or an allowed maximum data rate and allowed transmission timing. In step 110, the UE determines the TF of the E-DCH (E-TF) based on the scheduling grant. The UE then transmits E-TF information to the Node B in step 112, and uplink packet data on the E-DCH as well in step 114.
The Node B determines whether the E-TF information and the uplink packet data have errors in step 116. In the presence of errors in either of the TF information and the uplink packet data, the Node B transmits a negative acknowledgement (NACK) signal to the UE on an acknowledgement/negative acknowledgement (ACK/NACK) channel, whereas in the absence of errors in both, the Node B transmits an acknowledgement (ACK) signal to the UE on the ACK/NACK channel in step 118. In the latter case, the packet data transmission is completed and the UE transmits new packet data to the Node B on the E-DCH. On the other hand, in the former case, the UE retransmits the same packet data to the Node B on the E-DCH.
For efficient scheduling under the above-described environment, the Node B receives scheduling information about buffer occupancy and power status from UEs. Based on the scheduling information, the Node B allocates low data rates to remote UEs, UEs in a bad channel status, and UEs having data with a low service class and high data rates to nearby UEs, UEs in a good channel status, and UEs having data with a high service class. In this context, a need exists for developing a technique for transmitting and receiving a scheduling grant, which can be an Absolute Grant (AG) indicating the absolute value of an allowed maximum data rate for a UE or a Relative Grant (RG) indicating up/hold/down from the previous allowed maximum data rate.